The Study Of Ni-Mo-W Catalysts Prepared By Complete Liquid-phase Method For Ultra-deep Hydrodesulfurization Of Diesel Fuel

Sulfur compounds are one of the most common impurities presented in diesel fuel. Combustion of the sulfur diesel fuel leads to the emission of SOx and sulfate particulate matter(SPM) which results to generate PM2.5 and the SOx gases react with water in the atmosphere form sulfates and acid rain which would cause great damage to the animals and plants. Besides, the sulfur in diesel fuel can shorten the life of the diesel engine due to corrosion and significantly impair the exhaust gas purifying device. The issue of diesel ultra-deep desulfurization is becoming more and more attracting because the crude oil is becoming heavier in density and containing more sulfur than before, which will lead to the higher sulfur content in diesel fuel, while the statutory sulfur content in most countries are becoming even lower for the increasing environmental concern.Hydrodesulfurization(HDS) is an important technology for the production of clean diesel fuel, and previous research of diesel HDS mostly focused on the fix bed reactor. The fix bed technique has shortcomings of high temperature and pressure, high equipment investment and operating cost, and poor heat transfer efficiency, which will lead to local overheating and furthermore the coking and deactivation of catalysts. On the contrary, slurry bed reactor has the advantages of homogeneous heat distribution, higher heat transfer efficiency, operating at more constant temperature, replacing catalyst on-line without factory shutdowns, which will reduce costs enormously. And it has become an important development direction in the HDS field in recent years. Aiming at the slurry bed reactor, our research group proposed a novel preparation method of the catalyst-complete liquid-phase method, whose main innovation lies in preparing the catalysts directly from solution to the slurry. However, the HDS catalysts prepared by the method exists poor repeatability and dispersion, which was caused by the raw material of Ni(NO3)2·6H2O, whose property is easy to be interfered by environment and easy agglomeration of the element of Ni. Aiming at the problems and defects of the previous Ni-Mo catalysts, Ni-Mo-W tri-component catalysts were synthesized by the complete liquid-phase method to improve the hydrogenation performance and desulfurization activity of the catalysts.On the basis of previous achievements, decreasing the content of Ni element and increasing the second active component W and additive P to study the desulfurization performance of the prepared catalysts. Furthermore, the adding method of W and exposure time in preparation process of sol-gel precursors were also investigated. The catalysts with higher activities were retested to check the reproducibility of the preparation process and characterized by several techniques including X-ray diffraction(XRD), nitrogen adsorption(BET), X-ray photoelectron spectroscopy(XPS), temperature-programmed desorption of NH3(NH3-TPD) and scanning electron microscopy(SEM). Together with the results of diesel desulfurization, the main conclusions are as follows:1 Addition of the second active component can improve the catalytic activities and stability, and the catalyst with the mole ratio of Ni/Mo/W=1:2.6:0.45 had the highest sulfur removal effect. The specific surface area increased and optimal pore sizes were shaped because of the addition of W. The best adding method of W is before the adjunction of Mo and after the adjunction of Ni.2 A proper amount of P can promote the desulfurization performance of the catalysts significantly, and the optimum weight of P is 1 wt%. The prepared catalyst can remove the sulfur content of the diesel fuel to lower than 10 ppm with the desulfurization rate of 97.9%.3 The addition of P auxiliary can increase the surface acidity, decrease the interactions of carriers and active phases and thus increase the sulfidation degree, while overdoses of P will lead to the agglomeration of active components and harm to the desulfurization activity of the catalysts.4 Exposure time in the process of sol-gel precursors have an important effect on the quality of sol, and proper amount of supernatant does good to the aging of sol to gel, and can cross-link to a stable precursors structure.